The results of genetic studies of R6K Tra1- and R6Kdelta[Sm1] mutants of R6K plasmid and those of heteroduplex analysis of DNAs have shown that DNA of this drug-resistant factor contains three loops flanked by the inverted repeats. The latter are designated as IR1, IR2 and IR3 and are of 50, 100 and 120 nucleotides in size respectively. IR1 is inserted into the loop flanked by IR2. Loops with these two repeats are located in major EcoR1 fragment, IR3 having been found in minor EcoRI fragment of the plasmid. The evidence obtained from the analysis of heteroduplex R6K/RSF2124 has shown that the loop with IR1 is corresponding to transposon Tn3. The extent of the deletion deltaSm1 indicates that IR2 may be a part of a transposon bearing the resistance to streptomycin. By comparing present data with those obtaine from the analysis of the RSF1040 factor of DNA replication initiation sites (Grosa et al., 1976), it has been suggested that the loop with IR3 represents a transposon with replicative functions (TnRep). The deletion of the mutant plasmid R6Kdelta[Sm1] (7.2 . 10(6) daltons in size) which affected one of the EcoRI sites not only confers the sensitivity to streptomycin but enhances also the efficiency of conjugational transfer and results in the loss of the R6K ability to bring about integrative suppression and to inhibit the fertility of the plasmids from IncP and IncN groups. The deletion mutant proved to have lost the property of incompatibility with the initial plasmid R6K and with itself.